1. Field of the Invention
The present invention relates generally to heater wire and more particularly relates to heater wire having an integral sensor wire and an improved controller therefor.
2. Description of the Prior Art
Specialized wire for performing electro-thermal conversion, commonly referred to as heater wire, is well known in the prior art. Heater wire is used in a variety of applications including measurement systems, industrial heating processes, and in textile articles, such as electric blankets.
When used in personal textile articles, such as electric blankets, it is critical that the heater wire is used in conjunction with over-heat protection to reduce the risk of fire. Several attempts have been made in the art of electric blanket design to incorporate such protection. These designs attempt to balance user comfort and user safety. To achieve maximum comfort to the user, the heater wire should be permitted to operate as close to the maximum temperature of the article as permitted. To achieve maximum safety, the heater wire must not exceed the maximum temperature or an enhanced risk of fire results.
FIG. 1 illustrates a simplified schematic diagram of a heater wire 2 formed with a plurality of thermostats 4 interposed therein. The heater wire 2 is formed from a resistive material and generates thermal energy when a current is passed through it. The thermostats 4 are temperature sensitive switches, such as bimetallic switches, which are normally closed and open only in response to the detection of a predetermined temperature limit. This configuration, which is well known in the art, provides discrete points of over temperature sensing and protection. However, the protection offered by this configuration is limited in that only a certain number of points of protection are distributed throughout the length of the heater wire 2. Further, each thermostat 4 requires two electrical interconnections to the heater wire 2. These connections tend to increase manufacturing costs and decrease product reliability. In addition, the thermostats have a relatively high thermal mass which results in a slow response to rapid temperature changes. As a result, the temperature of the heater wire tends to overshoot the desired temperature.
To overcome the problems associated with the heater wire embodiment illustrated in FIG. 1, heater wires have also been formed which are thermally self limiting. An example of such a self limiting heater wire is disclosed in U.S. Pat. No. 4,309,597 to Crowley. Such a device is illustrated in the simplified schematic diagram in FIG. 2. The heating wire of FIG. 2 includes a pair of conductors 6, 8 and a positive temperature coefficient (PTC) resistive material 10 disposed therebetween. The PTC material 10 forms a distributed resistance between the conductors 6, 8.
When a constant voltage is applied across the two conductors 6, 8 a current flows through the PTC material 10 and heat is generated. As the PTC material 10 is heated, the resistance of the PTC material 10 increases and the current is reduced accordingly. In this way, the heater wire effectively self regulates to provide over temperature protection. This embodiment of a heater wire eliminates the manufacturing problems associated with the connection of multiple thermostats within a resistive heating wire. However, the PTC material 10 is typically a high carbon content material and is extremely flammable in the presence of electrical arcing. Therefore, should cracks arise in the PTC material 10 across which electrical arcing occurs, a high risk of fire is presented.
U.S. Pat. No. 4,503,322 to Kishimoto et al. discloses a further embodiment of a self regulating heater wire. The heater wire formed in accordance with the '322 patent is shown schematically in FIG. 3. This heater wire includes three conductors 6, 8, 12 and two thermal-resistive layers 10, 14. Two of the conductors 6, 8 and one thermal-resistive layer 10 function as a heater wire essentially as described in connection with FIG. 2.
The second thermal-resistive layer 14 is interposed between one of the heater wire conductors 6, 8 and the third conductor 12, which is used as a sensor wire. As heat is generated in the first layer 10, the resistance of the second thermal-resistive layer 14 changes in response. This change in resistance between the conductors 8, 12 is indicative of the temperature of the heater wire and can be used to regulate the heat generated by the heater wire. While this configuration of a heater wire provides a sensor wire which enhances the controllability of the heater wire, the fundamental problem of the high flammability due to the high carbon content of the thermal-resistive layers is still present.